Change the information that is displayed when shelling out

In `warn` mode, Omnibus will only display warnings/deprecations; you will not see any build commands or output. In `info` mode, Omnibus will display the commands and environment that are being used; you will not see the output from the build (unless it fails). In `debug` mode, Omnibus will display the commands, environment, and output (livestream) from commands.

/cc @opscode/release-engineers @sersut @danielsdeleo